SIRENIANS: CHARACTERISTICS, EVOLUTION, TAXONOMY, FEEDING - BIOLOGY - 2025

The sirenios (sirenians) are vertebrate animals that live in water and belong to the order Caudata. They are characterized by having gills on both sides of the neck, located between the eyes and their only legs, the front ones. Its body is elongated, similar to that of an eel.

Their diet is based mainly on insects and small invertebrate animals, although they also feed on plankton, moss, stems and leaves of aquatic plants.

Stan Shebs, from Wikimedia Commons

They are paedamorphic animals, since adults present a change in their phenotype and genotype as a result of the displacement of some traits from their ancestors. One of these characteristics are the gills, because they occur both in the larval stage and in adulthood.

Some species have a marked sexual dimorphism, males are generally larger than females. In the same way, they tend to have proportionally larger heads than females.

Communication

For the most part, sirenians are solitary animals, having few interactions with members of their species. Despite this, some species can use various techniques in order to evade predators.

They are able to vocalize sounds that intimidate the attacker. These can be howling, hissing, or a sound similar to that made by ducks.

Another option the sirenians take is to escape quickly, using their muscular tail. They may also decide to confront the predator, who could be given a painful bite, causing it to move away.

Because your eyes are so small, your vision is most likely not the primary sense you use to perceive your surroundings. As their habitat is bodies of water, they could be cloudy, have mud and a lot of vegetation, making their visibility much lower.

In order to orient themselves and locate their prey, the sirenians use their lateral line, which allows them to feel the vibrations that exist in the environment. This makes it easier for them to orient themselves and perceive how close they are to the prey.

General characteristics

Size and shape

In cross section, its body is rounded, covering approximately two-thirds of the total length. The rest is made up of a long tail, flattened vertically.

The greater mermaid (Siren lacertina) can measure from 50 to 90 centimeters. The lesser siren (S. intermedia), could have a body between 18 and 65 centimeters long.

As an adult, dwarf mermaids (Pseudobranchus) are typically 4 to 9 inches from their head to tail.

Skin

The color of the skin is usually dark, presenting dark brown, black, green or grayish-blue tones at the dorsal level. Sirenians that are lighter in color have brown or black spots.

Young sirenians have lines that extend from the neck to their distal end, the tail. Longitudinal markings can be seen in her eyes.

In the ventrolateral area, light areas are usually found that may change to reddish-orange or even yellowish tones. These may go away when they reach adulthood.

Newborns have a red or yellow triangle-like mark on their snouts. In the young the color is brighter, having a more mottled appearance than the adult species.

Rib furrows

Adult sirenians are distinguished from young ones by the number of costal grooves, which are lateral grooves extending along the body. The older sirenians have about 40 furrows, while the young ones have between 30 and 35 costal furrows.

Breathing

Sirenians are aquatic animals, which eventually come out of the water onto land, or perch on the leaves of plants found in the water.

Due to this behavior, they have external gills, to breathe in water. They also have primitive lungs, which allow them to exchange oxygen and carbon dioxide on earth.

In addition to this, research has shown that they are capable of breathing through the epidermis.

Evolution

The oldest fossil record is the Karauridae, an extinct group that lived in the late Jurassic period. The Chinese specimen Beiyanerpeton jianpingensis is considered a primitive antecedent of the salamander that lived in the Upper Jurassic.

Triassurus sixtelae shares two characteristics with salamanders: they are small in size and have a larval stage, due to their poor ossification. This species dates from the end of the Triassic, so it can be associated with the oldest record of a salamander.

Phylogenetic studies on the relationship of salamanders and other modern amphibians showed a close relationship with the Procera group.

The monophyly of the main groups of salamanders is distributed in 5 branches: Cryptobranchidae and Hynobiidae, Sirenidae, Salamandridae - Ambystomatidae - Dicamptodontidae, Proteidae and Rhyacotritonidae - Amphiumidae - Plethodontidae.

Molecular investigations placed the Sirenidae as a sister group to salamanders. The oldest member of the family Sirenidae is the genus Habrosaurus, which lived in the late Cretaceous. It was large in size with blunt teeth, suggesting that it fed on crustaceans and snails.

Taxonomy

Animal Kingdom.

Subkingdom Bilateria.

Infra-kingdom Deuterostomy.

Chordate Phylum.

Vertebrate Subfilum.

Infrafilum Gnathostomata.

Tetrapoda superclass.

Amphibian class.

Order Caudata

The family sirenidae is classified into 2 subfamilies:

Pseudobranchus

Members of this genus are aquatic, having greater activity at night. They have small forelegs, with three toes each. They lack hind limbs.

They have gills and inhabit North America from South Carolina to Florida. Some representatives are the southern dwarf mermaid (Pseudobranchus axanthus) and the northern dwarf mermaid (Pseudobranchus striatus).

Siren

The species of this genus live in semi-permanent or permanent bodies of water, such as lakes and ponds. It has paedomorphic characteristics, such as gills and suction feeding.

They only have front legs, with 4 toes on each. They are distributed in the southeastern United States and northeast Mexico. The greater siren (Siren lacertina) is one of the members of this species.

Feeding

Sirenians are most active at night. They are mainly carnivorous animals, although plant species, such as algae, have been found in the digestive system of some specimens. This leads researchers to argue that they may be omnivorous animals.

Their diet usually includes insects, spiders, mollusks, crustaceans, gastropods, small fish and crabs. They also consume amphibian eggs and larvae in their diet. They also occasionally eat algae and vascular plants.

They are opportunistic feeders, being also considered as medium level predators of some habitats, since they feed on insects and other species of invertebrate animals. In this way, they are usually controllers in the population of other organisms in the food chain.

Due to its visual limitation, its small eyes, and its nocturnal habits, this animal uses some chemosensory strategies to locate its prey.

For example, they use the vomeronasal organ, an auxiliary olfactory structure to detect their prey in the cloudy and dense waters where it is found.

Digestive system

Some of the characteristics of the digestive system of sirenians are inconsistent with those of herbivores. Due to the dental structure, they do not chew, break or grind the invertebrates or plants they digest.

Despite the fact that their intestines are shorter than those of herbivores, research confirms that microbial fermentation occurs in the digestive process, typical of animals whose diet is exclusively plant-based.

The stomach of the sirenians is not very bulky. The back part of the intestine is enlarged, showing folds and the presence of an ileocolonic valve, responsible for maintaining the microbial bacteria that aid in digestion.

The intestines usually have symbiotic microbes, responsible for fermenting carbohydrates that could not be digested by enzymes. This occurs with cellulose, a component of vegetable fibers, which needs to be completely processed in order to release by-products, such as fatty acids.

These by-products are absorbed by the intestine and used by cells as an energy source.

Reproduction

Females are sexually mature at two years of age. Some data are not available on the specific characteristics of mating in sirenians, so this aspect is the subject of study.

The eggs are deposited in the mud, under the rocks or where the vegetation is thick, so that these environments become their protective elements.

The pose is usually done in groups, forming a kind of small grapes attached to each other. The number of these groups can vary, finding nests of 12 eggs and others with more than 200. The gestation of the eggs lasts about 2 months.

In relation to fertilization, there is controversy as to whether it occurs outside or inside the female's body. Some researchers argue that it is external, as no sperm samples have been found in the female's oviducts.

On the other hand, other specialists affirm that the laying site of the eggs prevents the male from fertilizing them externally. This leads them to maintain that fertilization is internal and that sperm is stored in the female's oviducts.

Behaviors

The members of the Sirenido order carry out some behaviors that can be classified as courtship. These rituals include chasing each other, waving the tail, and rubbing the head.

Once this is finished, the female unloads the eggs in the nest. After fertilization, the female leaves and the male remains taking care of the nest, built with mosses and leaves.

During the development of the eggs, the male drives away intruders, even biting them, in order to keep them away from the young.

Anatomy and morphology

Lungs

Sirenians can burrow into the muddy terrain of ponds, encasing themselves in a cocoon of mucus. In this way they prepare to survive long periods of drought. During this stage they can breathe with their small but functional lungs.

Gills

They have neotenic gills, which implies that even in the adult state, the animal has preserved the larval characteristic of this organ. Although in larvae the gills are small and not functional, in adults they are already fully developed.

The gills are external, with three agglomerated gill appendages on the front of the head, behind each eye.

In the event that the water changes its chemical characteristics, the larvae could reduce this organ to simple non-functional stumps.

Heart

Mermen share some characteristics with non-aquatic salamanders. However, they are differentiated by the fact that their heart has an interventricular septum, not present in exclusively terrestrial species.

Head

Its head has a rounded shape, ending in a kind of short trunk. The eyes are small and do not have lids. Its jaw is square and is reduced to free and mobile elements, which move ventrally with respect to the rest of the cranial structure.

Extremities

The hind limbs in the mermaid are absent. The front ones have 4 fingers and are reduced, with very little development of their muscles and bone structures. These animals lack a pelvic girdle.

These features on their legs have prevented them from colonizing habitats on land, however they can move along the bottom of some aquatic niches.

Lateral line

Sirenians have a lateral line of sensory organs that allow them to detect movement, pressure changes, or vibrations of the water. This helps it to orient itself and locate its prey.

Teeth

In his mouth there are no premaxillary or maxillary teeth. Only in some exceptions can some small teeth be seen on the palate and in the splenial bone, on the inside of their lower jaws.

This group of aquatic salamanders does not have teeth attached to the jaw via a flexible peduncle, so it is stated that they are not pedicellate.

Due to this, some specialists affirm that sirenians lack dentition, which is replaced by a corneal structure similar to a beak.

Habitat

Sirenians do not have, within their development, a terrestrial life stage. So they are almost exclusively found in aquatic environments.

Its habitat can be a body of water that has herbaceous plants, such as marshes. They are also found in swamps, canals, lakes, streams, and ponds. The presence of abundant vegetation in the habitat of the sirenians is important, because it allows them to hide from predators.

The hatchlings grow in the thick vegetation, gradually moving into deeper waters. Once adult, they spend most of their lives under sunken trunks, entwined between the roots and branches of the plant.

When water sources dry up or drop in level, sirenians sink into the muddy lake or stream bed, encapsulating themselves to prevent desiccation.

The species are usually widespread in southern and eastern North America, on the Atlantic coast plains from Virginia to Florida, including eastern Texas. They can also be found in northeastern Mexico, in regions such as Tamaulipas and northern Veracruz.

References

1. Wikipedia (2018). Sirenidae. Recovered from en.wikipedia.org.
2. Darren Naish (2016). The Biology of Sirens. Scientific american. Recovered from blogs.scientificamerican.com.
3. McKenzie, K. (2012). Siren lacertina. Animal Diversity Web. Recovered from animaldiversity.org.
4. Encyclopedia britannica (2018). Siren. Recovered from com.
5. GREGORY S. PRYOR, DONOVAN P. GERMAN, KAREN A. BJORNDAL (2006). Gastrointestinal Fermentation in Greater Sirens (Siren lacertina). BioOne. Recovered from edu.
6. ITIS (2018). Sirenidae. Recovered from itis.gov.